118 IBM Communications Server for AIX, V6: New Features and Implementation Scenarios
Technically, a Branch Extender poses as a network node with DLUR support
to computers or 3270 terminals in its local branch. At the same time it poses
as an end node to computers in the WAN. By impersonating an end node, a
Branch Extender gains the following advantages:
It does not receive topology data or directory broadcasts from the wide
area network. Instead, it lets a network node in the WAN choose routes for
computers in the branch. A Branch Extender relays data between the
branch and the WAN and enables HPR traffic to flow to computers in the
branch.
It can register all the branchs resources to a directory server in the APPN
backbone network so that no manual definitions are needed there.
Network efficiency is improved by notifying the central directory server of
each resources location, to greatly reduce or even eliminate broadcast
searches in the WAN.
Branch Extender also allows direct, peer-to-peer communication between
branches connected to the same APPN connection network (CN). Though a
connection network configuration is not required, Branch Extender provides
substantial benefit when the uplink and downlink networks are CNs, for
example a branch CN such as token-ring, Ethernet or FDDI connected via a
BrNN to a WAN CN such as ATM.
7.2 Topology databases
APPN networks consist of a backbone structure of network nodes
interconnected by TGs, known as intermediate-routing TGs, and TGs
connecting end nodes to adjacent network nodes, virtual routing nodes, or
other end nodes, known as endpoint TGs.
Information about the backbone structure of the APPN network is kept within
the network topology database, which resides on every APPN network node.
Information about endpoint TGs is contained within local topology databases,
which reside on every APPN node or LEN end node.
The primary use of local and network topology databases is to enable route
calculation when an LU residing in one APPN node wishes to establish a
session with an LU residing in another APPN node. The topology databases
enable topology and routing services in the network node (NN) server to
determine all possible routes between the nodes. The local topology
database contributes the end nodes TG, while the network topology
database supplies the information on network nodes and the TGs between
them.
Chapter 7. Branch Extender 119
7.2.1 Local topology database
The local topology database contains information on all of the transmission
groups (TGs) attached to the node. Every APPN node maintains a local
topology database. An APPN end node uses the information in its local
topology database to send local TG information to its network node server in
locate requests and replies. In a network node, the local topology database
includes information about the attached end nodes. In HPR networks, the
APPN end nodes TG information (carried in TG vectors) indicates whether
the nodes are HPR-capable.
The local topology database is created and maintained by the end node
Topology Database Manager (TDM). It is not saved across reboots and is
rebuilt when the node initializes. Entries in the local topology database are
created automatically when configuration services informs TDM about newly
activated or changed TGs. The operator updates the local topology database
through configuration services. The local topology database is searched by
TDM when it receives a query from route selection services or from session
services.
7.2.2Network topology database
Network nodes in an APPN network need to maintain topology information
about the location of other nodes in the network and the communications
links between them, and to forward this information around the network when
the topology changes. The information is maintained in a database called the
network topology database. As the network grows in size, the amount of
stored information and topology-related network traffic can become large and
difficult to manage. Topology database size is a function of the number of
NNs and links, as well as the frequency of link state changes.
Every APPN network node maintains a network topology database in addition
to its local topology database. The network topology database does not
include information on APPN end nodes, LEN end nodes, or the transmission
groups attached to them. It includes information only on network nodes in the
APPN network and the transmission groups interconnecting them.
Network nodes in an APPN network send one another topology database
updates (TDUs) over CP-CP sessions whenever a resource (network node or
a TG between network nodes) is activated or deactivated, or its
characteristics change. Only the current changes are included in the TDU.
Every network node receives the TDU containing the current change, so each
has the same view of the network. The network topology database is used by
the network node to select routes for sessions that originate at the LUs in it
and at the end nodes that it serves. Unlike the directory database, which is

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